1. Field of Invention
This invention relates to a surface light source device of side light type and a display employing the same, and more particularly, to a surface light source device of side light type and a display, which achieve a reduction of irregularities in luminance. The present invention is applied to a backlighting arrangement in a liquid crystal display panel, for instance, and provides a liquid crystal display of high display quality.
2. Related Art
It is a matter of common knowledge that a surface light source device of side light type provides a thin backlighting arrangement to illuminate a liquid crystal display panel. In general, the surface light source device of side light type has a light guide plate in the form of a plate-like member made of a light guiding material, and a primary light source including a rod-shaped light source to supply illumination light to the light guide plate through a lateral side thereof.
Illumination light emitted from the primary light source is introduced into the light guide plate through an end surface of the light guide plate. The introduced illumination light is deflected and then emitted toward a liquid crystal display panel through one of major surfaces of the light guide plate. Since the primary light source is arranged on the lateral side of the light guide plate, it is possible to easily provide a device of thin type on the whole.
A well-known light guide plate employed in the surface light source device of side light type as described above is classified into a light guide plate of a type which is approximately uniform in thickness, and a light guide plate of a type which shows a tendency to decrease a thickness according as to remoting from the primary light source. In general, the light guide plate of the latter type emits illumination light more efficiently than the light guide plate of the former type.
FIG. 7 is an exploded perspective view showing a surface light source device of side light type employing the light guide plate of the latter type. FIG. 8 is a sectional view taken along a line A--A in FIG. 7. Referring to FIGS. 7 and 8, a surface light source device of side light type 1 has a light guide plate 2, and a primary light source 3 is arranged on the lateral side of the light guide plate. The surface light source device 1 further has a reflection sheet 4, a diffusible sheet 5, prism sheets 6, 7 functioning as light control members, and a protection sheet 8 of low diffusibility, and these components are laminatedly arranged together with the light guide plate 2 as shown in FIG. 8.
The primary light source 3 has a cold cathode tube (a fluorescent lamp) 9 and a reflector 10 surrounding the cold cathode tube. The reflector 10 has an aperture, through which illumination light is supplied to an end surface 2A of the light guide plate 2. The reflector 10 is made of a regular reflective or diffuse reflective sheet material or the like. The light guide plate 2 having a wedge-shaped section is made of an acrylic material (PMMA resin), for instance, by means of injection molding. The light guide plate 2 receives the illumination light from the primary light source 3 through one end surface serving as an incidence surface 2A.
In the light guide plate 2, the illumination light makes a propagation while undergoing repetitive reflection between a major surface (which will be hereinafter referred to as "slope") 2B, along which the reflection sheet 4 is disposed, and a different major surface (which will be hereinafter referred to as "emitting surface") 2C, along which the diffusible sheet 5 is disposed.
Every time reflection occurs, a component of light incident at an angle of not more than a critical angle is emitted through the slope 2B and the emitting surface 2C. Accordingly, the illumination light emitted through the emitting surface 2C makes a main propagation inclined toward the wedge end. This phenomenon is called emitting directivity.
Further, a diffusible surface 2D is formed on the slope 2B. The diffusible surface 2D is provided with diffusibility which steps up from the side of the incidence surface 2A toward the wedge end. The diffusibility is provided by means of coating the slope with diffusible ink containing a pigment consisting of magnesium carbonate, titanium oxide or the like, for instance. The degree of diffusibility can be adjusted according to a quantity of diffusible ink coated on the slope.
The slope 2B may be also provided with diffusibility by means of matting (roughening) the slope 2B, instead of coating the slope with the diffusible ink. In this case as well, such diffusibility as steps up from the side of the incidence surface 2A toward the wedge end is provided. Thus, matted surface areas of a rectangular shape, for instance, are formed in a distributed state at a certain or random pitch so that the density of distributed areas steps up from the side of the incidence surface 2A toward the wedge end.
The light guide plate 2 described above corrects a quantity of output light decreased in the vicinity of the wedge end, and makes the distribution in quantity of output light uniform. It is to be noted that the diffusibility described above is not so intensive as the light guide plate is allowed to lose the emitting directivity. That is, even if the diffusibility is provided to the slope 2B, the illumination light emitted through the emitting surface 2C is inclined to mainly toward the wedge end.
The reflection sheet 4 is made of a sheet-like regular reflective member consisting of metal foil or the like, or a sheet-like diffuse reflective member consisting of a white PET film or the like. Illumination light leaking out of the slope 2B is incident again onto the light guide plate 2 after having been reflected by the reflection sheet 4, resulting in improvement of efficiency of illumination light utilization.
The prism sheets 6, 7 are arranged to correct the emitting directivity of the light guide plate 2. The diffusible sheet 5 is arranged to prevent the diffusible surface 2D on the slope 2B from being visibly observed from above the emitting surface 2C, and further to make highlight and shadow or the like in each part of the light guide plate 2 illuminated with the illumination light less noticeable.
The diffusible sheet 5 diffuses the illumination light emitted through the light guide plate 2. The prism sheets 6, 7 are made of a light-transmitting sheet material such as polycarbonate. In each prism sheet, a surface (an outside surface) with its back to the light guide plate 2 is formed as a prism surface. The prism surface is composed of a large number of projections, which are triangular in sectional shape and run approximately parallel in one direction. In the shown case, the projections on the inside prism sheet 6 are oriented such as to run along the incidence surface 2A, while the projections on the prism sheet 7 are oriented such as to run in a direction orthogonal to the incidence surface 2A.
The prism sheets 6, 7 correct a main emitting direction of output light with a slope of each projection so as to emit the output light in a frontal direction of the emitting surface 2C. It may be also possible to use a so-called double-faced prism sheet having both surfaces respectively serving as prism surfaces.
In general, the surface light source device of side light type employing the wedge-shaped light guide plate and the prism sheets as described above may emit output light in the frontal direction more efficiently than a surface light source device of side light type employing a light guide plate which is substantially uniform in thickness.
The protection sheet 8 protects a surface of the prism sheet 7 from damage or the like, and relieves the directivity of output light corrected by the prism sheets 6, 7 to enlarge an angle of visual field. Accordingly, the illumination light may be emitted to a desired extent in the frontal direction of the emitting surface.
The surface light source device of side light type 1 (See FIG. 7) having the above basic structure has a frame 11. Various components including the light guide plate 2 and a liquid crystal display panel are arranged in sequence on the frame 11, and are supported thereto. The frame 11 is made of a white resin of high reflectance, which efficiently reflects illumination light, by means of injection molding. The frame 11 is composed of a thin plate-like side plate 11A to surround the lateral side of the surface light source device of side light type 1 and a bottom to close up the side plate 11A.
The bottom has a thin plate-like muntin 11B fringing the slope side of the light guide plate 2 and connected to the side plates 11A at a predetermined position. The muntin 11B of the frame 11 decreases deformation in molding. Further, in the frame 11, the muntin 11B projects downward at a predetermined position as shown in the drawing. A tapped hole is formed in this projected portion, and a drive substrate (not shown) of the liquid crystal display panel or the like is supported on the rear side by the use of this tapped hole, so that the structure of the frame secures sufficient strength and holds the total weight down.
In assembly, various components such as the reflection sheet 4, the light guide plate 2, the primary light source 3, the diffusible sheet 5, the prism sheets 6, 7 and the protection sheet 8 or the like are housed in sequence at predetermined positions in the frame 11. A rod-like support member 12 is arranged on the outermost side (on the side of the protection sheet 8), and the above components are supported more fixedly with this support member.
When observation of the above well-known surface light source device of side light type 1 is carefully made from above the emitting surface, irregularities in luminance resulting from partial intensification of a luminance level are sensed in an area corresponding to the muntin 11B of the frame 11. If it is possible to achieve a reduction of the above irregularities in luminance, output light from the surface light source device will be graded up, and the display employing this surface light source device will be improved in display quality.